<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <form name="loginForm" method="post" action="/login">
        <div>
            <label for="phoneInput">手机</label>
            <input type="text" name="phone" id="phoneInput" placeholder="请输入手机" required pattern="/^[0-9]{11}$/">

        </div>
        <div>
            <button type="submit">提交</button>
        </div>
    </form>
    <script>
        const oForm = document.forms["loginForm"];
        oForm.addEventListener('submit', function () {
            event.preventDefault();
            const phone = oForm["phone"].value.trim();
            // console.log(phone);
            // 手机号不能为空
            // required 是html5 input 才有的属性
            if (!phone.length) {// 为空
                console.log('手机号不能为空');
                return
            }

            if (!/^[0-9]{11}$/.test(phone)) {
                alert('手机号格式有误');
                return
            }

            this.submit();
        })
    </script>
</body>

</html>